improve recovery of invalid top level abstract modifier usage

pkg/front_end/lib/src/fasta/parser/parser.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.parser.parser;
 
 import '../fasta_codes.dart' show Code, Message, Template;
 
 import '../fasta_codes.dart' as fasta;
 
@@ skipping 252 matching lines @@
     token = _parseTopLevelDeclaration(token);
     listener.endTopLevelDeclaration(token);
     return token;
   }
 
   Token _parseTopLevelDeclaration(Token token) {
     if (identical(token.type, TokenType.SCRIPT_TAG)) {
       return parseScript(token);
     }
     token = parseMetadataStar(token);
-    final String value = token.stringValue;
-    if ((identical(value, 'abstract') && optional('class', token.next)) ||
-        identical(value, 'class')) {
+    String value = token.stringValue;
+    if (identical(value, 'abstract')) {
+      if (optional('class', token.next)) {
+        return parseClassOrNamedMixinApplication(token);
+      }
+      reportRecoverableErrorWithToken(token, fasta.templateExtraneousModifier);
+      token = token.next;
+      value = token.stringValue;
+    }
+    if (identical(value, 'class')) {
       return parseClassOrNamedMixinApplication(token);
     } else if (identical(value, 'enum')) {
       return parseEnum(token);
     } else if (identical(value, 'typedef') &&
         (token.next.isIdentifier || optional("void", token.next))) {
       return parseTypedef(token);
     } else if (identical(value, 'library')) {
       return parseLibraryName(token);
     } else if (identical(value, 'import')) {
       return parseImport(token);
@@ skipping 3855 matching lines @@
   }
 
   Token reportUnexpectedToken(Token token) {
     return reportUnrecoverableErrorWithToken(
         token, fasta.templateUnexpectedToken);
   }
 }
 
 // TODO(ahe): Remove when analyzer supports generalized function syntax.
 typedef _MessageWithArgument<T> = Message Function(T);